Our senses provide us with a vast amount of complex information about the world around us, from which we must select what is currently most relevant. Thus, perception is the level at which the largest problems of selection and integration arise. Perceptual tasks offer a wide variety of potential forms of conflict that may be monitored in different ways and may involve different forms of control. We will investigate four types of perceptual conflict that are likely to occur at different levels of visual processing, but that all involve the simultaneous presentation of multiple stimuli that compete for neural representation: 1. competition between molecular stimuli presented to the two eyes resulting in binocular rivalry; 2. competition among stimuli presented simultaneously in nearby locations; 3. competition between objects, and 4. competition between different attributes within a single object (task- switching). Our aim is to define the nature of the conflicts in different tasks and the brain systems that monitor it; the nature of the units between which conflict arises (e.g. objects or locations); the evidence that adjustments are made in the allocation of attention in response to changes in the degree of perceptual conflict; and the systems that control the allocation of attention to resolve the conflict. Thus far, there has been little evidence to implicate the anterior cingulate (AC) in perceptual conflict. For this reason, we will specifically test alternative hypothesis about the role of the AC in perceptual conflict. One possibility is that the AC monitors conflict at the decision or response level rather than at the perceptual level. Another possibility is that the AC can be activated by perceptual conflict, but only in cases where early attentional selection mechanisms fail.